The upside to your situation that I don't see mentioned yet, is that you're acquiring a broader skill base and more skills that you otherwise would. You don't just know how to write code, which all software developers do (or at least claim to be able to do on their resumes), you also have team leadership, QA, project management, product management and client support skills that the majority of your peers don't have and don't claim, which you can use to differentiate yourself from your competition when it's time to move on.
Speaking of differentiators, being able to do it as a one-man operation is a big one, as mentioned in another answer. It demonstrates that you can actually get results and aren't free-loading off others' work, and is worth highlighting on your resume. I'm a fan of putting "sole [job position]" down on my resume/CV to make that point.
I think it's also worth pointing out again that you have an opportunity to change things for the better in your current environment, which isn't common or easy in large, established environments, and looks great on a resume. Think ahead to your next job for a moment, and consider that anyone who's a serious candidate for a development position will have (or claim) core development skills. Being able to add that you created a code review process and a QA system and so on at your current employer is something that will set you apart from the bulk of the other candidates.
Far from hurting your future employability, this position is probably a career-booster. After a few years there, in addition to the core skills you have and will improve on, you'll have differentiating accomplishments and supporting skills that the majority of your colleagues won't.